Four properties in particular are of importance in actuators, specifically force, deflection, speed and construction space. In many actuator applications there are different working points at which either a high force or high speed is required. In the case of an actuator for ejecting tools in a machine tool, the actuator has to cover the deflection until making contact with the tool at a great speed, with no particularly great forces being required. As soon as the actuator is in contact with the tool, the requirement is precisely the other way around. Great forces are required in order to be able to eject the tool. However, no great speed is required when the actuator deflection necessary for this purpose is very small. Two required modes therefore arise for the actuator. A speed mode and a force mode. Such a concept with these two modes is also used ever more frequently in robotics.
Use is customarily made of a two-stage transmission which provides a possibility of switching over between the two modes, specifically the speed mode and the force mode. The torque/force surges during the switching over, in particular under load, are disadvantageous here. [1] discloses a linear actuator which counteracts the problem with the aid of a transmission and an additional motor. ([1]: A. Girard and H. Asada—A Two-Speed Actuator for Robotics with Fast Seamless Gear Shifting, 2015 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS)). The complexity and the power density of such a system disadvantageously still need a high degree of optimization.
As alternatives, different actuator principles, such as, for example, cable drives, which can be used for realizing two different modes are customarily provided. For example, twisted cables have, inter alia, a nonlinear transmission ratio, and therefore twisted cables can also be used by means of an additional rotation for higher forces from same motor unit. The advantage of such a solution includes lower losses. However, the two modes are coupled to each other via a hysteresis-affected relaxation process. In order to counteract this effect, researchers have developed a clutch mechanism with an additional motor unit. (see [2]: Y. J. Shin, H. J. Lee, K.-S. Kim, S. Kim,—“Dual-Mode Twisting Actuation Mechanism with an Active Clutch for Active Mode-Change and Simple Relaxation Process”, 2015 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS)). Such a system likewise increases the complexity of the overall system. The control of twisted cables and the nonlinearities associated therewith continues to be a research topic.